1. Field of the Invention
This invention relates to data conversion. The invention is particularly directed to analog to digital converters (ADC) and digital to analog converters (DAC). The invention is especially directed to the minimisation of error in data converters of these kinds.
2. Description of the Prior Art
Multibit oversampled analog to digital (ADC) and digital to analog (DAC) data converters are an attractive alternative to their single bit counterparts because of their reduced quantisation error and the relative ease of achieving loop operation which is stable and free of spurious tones. Recent work on sigma delta (.SIGMA..DELTA.) oversampled data converters has shown how averaging techniques can be used to extend the overall accuracy beyond the limits set by process-induced mismatch between the analog segments in the multibit converter. These averaging schemes in effect "noise-shape" the mismatch error away from the passband of interest. The effect of this noise shaping is illustrated qualitatively in FIGS. 1 and 2. FIG. 1 shows, on a plot of signal amplitude against frequency, the effect of segment mismatch errors, which result in harmonics of the signal to be converted being erroneously added to the passband of the converter. FIG. 2 shows the desired noise shaping effect, where the errors have been moved to a higher frequency and no longer interfere with the signal passband.
More recently, second order shaping of the mismatch error has been described, in which a multi-strand sigma delta (.SIGMA..DELTA.) loop is employed to noise-shape a vector of quantisation errors. Each strand of the loop controls a segment in the multibit DAC and applies a noise-shaping filter to its component of the quantisation error vector.
The term "noise-shaping" in the sigma-delta converter art has been used in reference to increasing the sampling rate to move the quantisation error signal away from the frequency of interest. In the present specification, the term "noise-shaping" is also applied to techniques directed to overcoming the impact of errors arising out of manufacturing tolerances in the components of sigma-delta systems, and is to be understood accordingly.